1. Field of the Invention
The present invention relates to a magneto-optical disk and a method of reproducing the data recorded on the same.
2. Description of the Related Art
An optical disk, in which data can be optically recorded and reproduced by use of a light beam such as a laser beam, is quite useful and advantageous as one of large capacity and high density recording media for digital data.
There are various types of optical disks. One type is so called a read only memory (ROM) type optical disk, such as a CD-ROM (compact disk ROM), in which the data can be only read by the ROM type reproducing apparatus, such as a CD player, at the user side. Another type is so called a write once read many (WORM) type, in which the data can be written only once and can be read many times by the WORM type reproducing apparatus at the user side.
Such an optical disk of ROM or WORM type etc. is provided with a thin transparent substrate of a disk shape. The digital signal is recorded on the optical disk in a form of the row of pits in a physical concave/convex manner in a size of micron order. The pits are formed on a predetermined track in a recording layer of the optical disk, with a pitch of about 1.6 .mu.m in a spiral shape, by irradiating a light beam which is controlled to form a focused light spot at a prescribed position on the track, for example.
In the ROM or WORM type optical disk reproducing apparatus, a reproducing light beam is controlled by a focusing servo-mechanism and a tracking servo-mechanism etc., so as to be focused on the row of pits with a spot diameter not greater than 1 .mu.m, for example.
Accordingly, by detecting the light intensity of the reflected light from the disk, which depends on the existence of the pits, the digital data on the ROM or WORM type optical disk can be reproduced.
Especially, in such a reproducing operation, the light beam is irradiated to the pits through the transparent substrate. Accordingly, since the diameter of the light beam at the surface of the substrate is relatively large compared with the diameter of the focused light spot at the pits, the undesirable effect of a dust or scratch on the surface of the substrate is effectively reduced.
As another kind of the optical disk, there is a magneto-optical disk, in which the data recording operation is performed by turning the direction of the magnetization of the magnetic film with the perpendicular magnetic anisotropy formed on a transparent substrate. This magneto-optical disk is provided with a recording film made of a rare earth transition metal alloys, for example. Such a magneto-optical disk is useful and advantageous since the content of the recorded data can be changed in many times at the user side.
Reproducing the data on such a magneto-optical disk is performed by a magneto-optical disk recording/reproducing apparatus by use of the magneto-optical Kerr effect or the Faraday effect.
Namely, when a linearly polarized light beam is incident on the magnetic film with the perpendicular magnetic anisotropy, the polarization plane of the reflected light or the transmitted light is rotated by a certain degrees with respect to the direction of the polarization plane of the incident light.
Accordingly, by detecting such a rotation of the polarization plane, which depends on the direction of the magnetization of the recording film, the digital data on the magneto-optical disk can be reproduced.
As described above, the magneto-optical disk can not be reproduced by the ROM or WORM type reproducing apparatus, since the detection object of the magneto-optical disk is the above mentioned rotation of the polarization plane enhanced by the Kerr effect or Faraday effect, while the detection object of the ROM or WORM type optical disk is simply the light intensity of the reflected light.
That is to say, the magneto-optical disk can be only reproduced in the presence of an exclusive magneto-optical disk reproducing apparatus, having no compatibility with the aforementioned ROM or WORM type optical disk reproducing apparatuses. This is a great disadvantage since the ROM or WORM type optical disk reproducing apparatuses are quite useful and can be well popularized in the users.
In addition, since such an exclusive magneto-optical disk reproducing apparatus deals with the linearly polarized light beam, a relatively large number of optical components are required, while the accuracy in the optical arrangement thereof is also critically requested, resulting in an increase of the total cost and size of the apparatus.
By the way, as for such a kind of magneto-optical disk, a signal detecting method can be also theoretically effected by use of the circular dichroism effect of a magnetic material. The circular dichroism effect is such an effect that the intensity and phase of the light is changed due to the direction of the magnetization of the magnetic material when a circularly polarized light beam is incident on the magnetic material.
It is reported in the Japanese Applied Magnetic Academy Report, 12, 1988, one example of such a detecting method using circular dichroism effect, in which a garnet film is utilized as a recording film and a differential signal at a bit boundary is detected. However, the circular dichroism effect by this reported method is very small especially in case of making a recording film from the rare earth transition metal alloys, so that it is very difficult to detect the corresponding signal. Accordingly, this method can hardly be put into practical use.